A variety of telecommunications private exchanges and, in particular, telephone private branch exchanges are known that, in addition to being capable of actual switching control, are also capable of control or realization of additional events that proceed beyond the actual switching event. Such additional functions are generally referred to as performance characteristics, whereby a great number of different performance characteristics are known particularly for "voice" communication service. Included among such performance characteristics are, in particular, the presentation of user instructions on an optical display means, call back in the no answer or busy condition of the called extension and busy override.
The performance characteristic of "busy override" enables extension subscribers to connect into the conversation and to speak during a call given a busy extension or a busy attendant's position. In busy override, a trunk offering tone is transmitted and all three participants are interconnected to form a conference. Busy override is not possible, for example, for extensions protected against busy override and for terminals with data protection. A performance characteristic is also known that is referred to as "displacing". The performance characteristic of "displacing" is that a connection in a switching system can be disconnected in favor of another connection having higher priority. It can be provided that the disconnection is indicated to the disconnected subscriber locations on the basis of a busy signal (Oden, Nachrichtenvermittlung, R. Oldenburg-Verlag Munich, 1975, ISBN-3-486-39902-2, page 78).
The extent of the exchange traffic from or to the private branch exchange is dependent on the number of what are referred to as outside trunk relay repeaters or trunk circuits with the respective exchange lines. As a rule, the number of extensions is greater than the number of exchange lines in private branch exchanges. In private branch exchanges that have more extensions than exchange lines, one or more extensions are excluded from outgoing exchange traffic as well as from incoming exchange traffic when all trunk circuits are occupied. The user of an extension who wishes to produce an exchange connection may have to repeatedly dial a traffic discrimination digit given a heavy usage of the private branch exchange with outgoing and/or incoming exchange traffic or given a high demand for exchange connections or may have to press a ground key in order to be able to ultimately produce the desired exchange connection.
A product brochure of Siemens AG "HICOM 600 System Product Data", Order No. A19100-K3161-G430-01-7600, discloses a communication system wherein external lines can be pre-noted (page 34 of the product data). When a subscriber receives the busy signal when attempting to seize an external line, he can make a queuing request for the next line to become free insofar as the appertaining extension subscriber has at least local authorization.